The present invention relates to glass compositions for making glass fibers, and more particular to glass compositions having lowered liquidus and forming temperatures.
The most common glass composition for making continuous glass fiber strands for textiles and glass fiber reinforcements is xe2x80x9cExe2x80x9d glass. The requirements as to what type of composition constitutes an E-glass composition are included in ASTM D578-98. An advantage of using E-glass is that its liquidus temperature is well below its forming temperature, i.e. typically greater than 56xc2x0 C. (100xc2x0 F.) and generally between 83 to 111xc2x0 C. (150 to 200xc2x0 F.). As used herein, the terms xe2x80x9cforming temperaturexe2x80x9d, xe2x80x9cTFORMxe2x80x9d and xe2x80x9clog 3 forming temperaturexe2x80x9d mean the temperature of the glass at which the viscosity of the glass is log 3, or 1000 poise, and the terms xe2x80x9cliquidus temperaturexe2x80x9d and xe2x80x9cTLIQxe2x80x9d mean the temperature at which solid phase (crystals) and liquid phase (melt) are in equilibrium. The difference between TFORM and TLIQ, referred to herein as xe2x80x9cdelta Txe2x80x9d or xe2x80x9cxcex94Txe2x80x9d, is a common measure of the crystallization potential of a given melt composition. In the glass fiber forming industry, xcex94T is typically maintained at a temperature of at least 50xc2x0 C. (90xc2x0 F.) in order to prevent devitrification of the molten glass during a glass fiber forming operation, and in particular in the bushing area.
Boron and fluorine containing glass were developed to meet these operating conditions. More specifically, the boron and fluorine were included in the glass batch materials to act as fluxes during the glass melting operation. In particular, E-glass can include up to 10 wt % B2O3 and up to 1.0 fluoride (see ASTM D 578-00 xc2xa74.2). However, these materials are volatilized during melting and boron and fluorine emissions are released to the atmosphere. Since boron and fluorine are considered pollutants, these emissions are closely controlled by environmental regulations, which in turn requires careful control of the furnace operations and the use of expensive pollution control equipment. In response to this, low boron and/or low fluorine E-glasses were developed. As used herein, xe2x80x9clow boronxe2x80x9d means that the glass composition is no greater than 5 weight percent (wt %) boron, and includes boron-free glass, and xe2x80x9clow fluorinexe2x80x9d means that the glass composition is no greater than 0.30 wt % fluorine, and includes fluorine-free glass.
For additional information concerning glass compositions and methods for fiberizing the glass composition, see K. Loewenstein, The Manufacturing Technology of Continuous Glass Fibres, (3d Ed. 1993) at pages 30-44, 47-60, 115-122 and 126-135, and F. T. Wallenberger (editor), Advanced Inorganic Fibers: Processes, Structures, Properties, Applications, (2000) at pages 81-102 and 129-168, which are hereby incorporated by reference.
Because the actual glass fiber forming operation is conducted at high temperatures, there is high energy usage associated with its production, along with associated high energy costs. In addition, the high temperatures accelerate the degradation of the refractory used in the glass melting furnace, as well as the bushings used to form the fibers. The bushings include precious metals that cannot be recovered from the glass as the bushings corrode. It would be advantageous to produce the glass fibers at the lowest possible forming and liquidus temperatures so as to reduce the energy usage and costs and thermal load on the furnace refractory and bushings, while at the same time provide the xcex94T required to ensure an uninterrupted glass fiber forming operation. Reducing the forming and liquidus temperatures of the glass compositions can also result in environmental benefits, such as but not limited to, a reduction in the amount of fuel required to generate the energy necessary for the fiber forming operation, as well as a reduction in the flue gas temperature. In addition, it would be advantageous if the glass compositions are low fluorine and/or low boron compositions so as to reduce or eliminate the environmental pollutants associated with these materials.
The present invention provides a low boron content glass fiber forming composition that has a forming temperature of no greater than 1240xc2x0 C. (2262xc2x0 F.), a xcex94T of at least 50xc2x0 C. (90xc2x0 F.), and a ratio of SiO2 to RO (i.e. CaO+MgO) of no greater than 2.35. In one nonlimiting embodiment of the present invention, the glass composition has a silica content of no greater than 59 weight percent. In another nonlimiting embodiment of the invention, the glass composition is boron-free.